If you’re using blueprints, all you have to do is to make sure the Steam Online Subsystem plugin is enabled and that you’ve added your Steam product ID in the .ini file. When enabling the Steam Online Subsystem, the .ini file will be updated with some strings necessary for it to run.
Once you’ve set up your account with Steam and added your product you can upload your build to their server via a command line tool from the SDK. It’s an absolutely terrible tool where basically anything can go wrong, but there is plenty of information on Steamworks as well as videos. Follow them very carefully, but one thing that isn’t mentioned is that you cannot use spaces in the path to your build, and this is a very common reason for it to fail. The tools won’t tell you what’s wrong, it will just say something along the lines of “Error: Something went wrong.”.